The present invention relates to a printing press including inkjet heads for spraying ink on a sheet.
For example, Japanese Patent Laid-Open No. 2013-240989 (literature 1) discloses an example of a digital printing apparatus including inkjet heads. The digital printing apparatus disclosed in literature 1 includes an ink drying device for drying ink applied, by the inkjet heads, to a sheet to be printed. This ink drying device is controlled based on the convey state of the sheet. The ink drying device irradiates the sheet with light such as infrared or ultraviolet rays to dry ink by thermal energy. This digital printing apparatus executes printing by raising the surface temperature of the sheet to a predetermined temperature.
The digital printing apparatus disclosed in literature 1, however, poses a problem that infrared or ultraviolet rays for drying ink heat the surface of a printing cylinder for conveying the sheet, which excessively raise the surface temperature of the sheet. If the surface temperature of a sheet becomes too high, the viscosity of ink changes, thereby degrading the image quality of a printing product.
This problem can be solved to some extent by cooling the printing cylinder by a water or oil cooling device. However, this type of cooling device is difficult to control since the output is very large, and the device is large in size. Furthermore, since the rise time is long and a coolant may leak, the apparatus is difficult to deal with. Therefore, it is required to readily cool the printing cylinder without using such type of cooling device.